Spectroscopy refers to the use of multi-wavelength radiation to non-invasively probe a variety of samples to determine the composition, health, or function of those samples. Prior art spectroscopy is done with filtered white light sources. For example, a white light source emits a broadband radiation, which is filtered with a tunable monochromator comprising a rotating grating and slit to generate narrowband radiation, which probes a sample. Diffuse reflected radiation is then detected by an optical detector. By tuning the monochromator, it is possible to construct a spectrum of the reflected radiation, which provides non-invasive information about the sample.
Although it enables spectral measurements over a wide wavelength range, the prior art white light spectrometer suffers from a number of limitations. First, the filtered white light source has a weak signal to noise ratio. Second, the grating-based system has critical intra-system mechanical alignments, and contains moving parts, leading to a bulky and complex system with slow measurement times. Lastly, some applications employ frequency domain measurements, which are not presently possible with white light sources since white light sources cannot be easily modulated at the required 100 Mhz to 3 Ghz rates.
One prior solution to these problems is to replace the white-light source with a tunable laser. This approach eliminates the rotating grating since the laser provides a source of tunable narrow-band radiation, which requires no further filtering. However, prior art tunable semiconductor lasers are typically limited in tuning range to less than 100 nanometers (nm). For ultraviolet spectroscopy tuning, the source is particularly difficult.